1. Field of the Invention
The present invention relates to a combined cycle power generation plant including a gas turbine plant and a steam turbine plant.
2. Description of the Prior Art
A combined cycle power generation plant is a power generation system including a gas turbine plant and a steam turbine plant. A high temperature thermal energy is utilized by the gas turbine, and a low temperature thermal energy is utilized by the stream turbine. Thus, the thermal energy is recovered and effectively used.
In such a combined cycle power generation plant, research and development thereof has been done with emphasis on how much the high temperature thermal energy can be increased for enhancing the thermal efficiency of the combined cycle power generation plant.
However, there must be provided a cooling system for cooling the gas turbine after the high temperature thermal energy has been utilized. Air has been used as a cooling medium in this cooling system.
However, when air is used as the cooling medium, even if the high temperature thermal energy is attained, there are problems which prevent the thermal efficiency from being enhanced. These problems include a power loss at an air compressor used for elevating pressure of air which is used for cooling the gas turbine. These problems also include lowering of the mean temperature of a gas flowing in passages in the gas turbine, when the gas is mixed with the air which is used for cooling the gas turbine. This lowering of the mean temperature results in a lowering of energy contained in the gas.
In order to resolve these problems and to enhance the thermal efficiency, a cooling system using the stream s the cooling medium, in place of air, has been disclosed.
Japanese laid-open patent No. Hei 05(1993)-163960 discloses an idea to employ steam as the cooling medium of the gas turbine, but various problems exist therewith which must be resolved.
Japanese laid-open patent No. Hei 05(1993)-163960 discloses only a basic idea of using steam as the cooling medium. In this regard, steam is supplied into a hot gas turbine from a certain steam source and flows through portions to be cooled. Then, the steam is led into a recovery portion after it has cooled the gas turbine. However, no mention is made of particular consideration of problems such as pressure loss of the steam during cooling of the gas turbine.
That is, steam cooling is still in a stage of trial and error, wherein no satisfactory solution to the aforementioned problems has been realized.
The prior remains a basic idea, as mentioned above, wherein cooling steam passages are formed in series so as to communicate with one another along an entire portion of the gas turbine to be cooled, i.e. a stationary system or a rotating system.
That is, the stationary system to be cooled is constructed such that the cooling steam flows in series through a first stage stationary blade and then a second stage stationary blade. The steam becomes heated as it flows through these blades, and then it flows to a recovery portion.
In such a construction, there is caused a large pressure loss at the stationary system to be cooled, and accordingly, various restrictions are placed on the upstream side and the downstream side of the stationary system, which make designing and manufacturing more restrictive. Also, operation of the gas turbine is restricted, and plant efficiency is reduced. That is, if pressure of the steam is reduced when it is recovered, after cooling of the stationary system, output from the steam turbine is accordingly reduced, which results in lowering of plant efficiency.
On the other hand, if a predetermined pressure is to be obtained continuously at the steam turbine, with this pressure loss being considered in advance, a control valve having a capacity larger than is necessary must be provided. This means an increase of cost in designing and manufacturing, as well as employment of an impractically large control valve.